1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a bipolar transistor formed in a semiconductor substrate or the like capable of a high precision measurement of a current amplification factor .beta..
2. Description of the Related Art
In a semiconductor device including bipolar transistors, a current amplification factor .beta. is a parameter resulting from a manufacturing process. That is, even when the manufacturing process is accurately carried out, the current amplification factor .beta. greatly fluctuates. For example, the current amplification factor .beta. may often be increased to twice a target value or may be often be decreased to half the target value.
Therefore, in a testing stage of a wafer, a measurement of a current amplification factor .beta. is performed upon one or more dummy bipolar transistors. As a result, if the measured current amplification factor .beta. is deviated from an optimum range, this entire wafer is determined to be defective and is scraped. In view of this, it is important to accurately measure a current amplification factor of a dummy bipolar transistor.
In a first prior art dummy bipolar transistor circuit, three pads are provided for a base, an emitter and a collector, respectively, of a dummy bipolar transistor. Then, probes are contacted to the pads, so that a base voltage V.sub.1 and a power supply voltage V.sub.cc are applied to the base and the collector, respectively. Thus, a base current I.sub.B and a collector current I.sub.C are monitored and a current amplification factor .beta. is calculated by EQU .beta.=I.sub.C /I.sub.B
This will be explained later in detail.
In the first dummy bipolar transistor circuit, however, since the collector current I.sub.C is exponentially dependent upon the base voltage V.sub.1, the base voltage V.sub.1 applied from the exterior needs to be very precise. Also, since a parasitic oscillation may occur due to a high frequency operation of the bipolar transistor, it is impossible to precisely measure the current amplification factor .beta.. In this case, the frequency f of the parasitic oscillation can be represented by ##EQU1##
where L is a parasitic inductor of one probe; and
C is a parasitic capacity of one pad. For example, if L=10 nH and C=1 pF, then the frequency f is 1.6 GHz, which enables an oscillation in the newest bipolar transistor having a cut-off frequency of tens of GHz.
In a second prior art dummy bipolar transistor circuit, first and second resistors are added to the first prior art dummy bipolar transistor circuit. That is, the first resistor is connected between the base and the collector of the dummy bipolar transistor, and the second resistor is connected between the base and the emitter of the dummy bipolar transistor. Thus, the base voltage is self-biased by the first and second resistors without application from the exterior. In this case, the current amplication factor .beta. is calculated by ##EQU2##
where R.sub.1 is a resistance value of the first resistor;
R.sub.2 is a resistance value of the second resistor;
V.sub.CC is a voltage at the pad connected to the collector;
V.sub.1 is a voltage at the pad connected to the base; and
I.sub.CC is a current flowing through the third pad. This will be explained later in detail.
In the second prior art dummy bipolar transistor circuit, however, since the current amplification factor .beta. is dependent upon an absolute value of the second resistor whose precision is about several tens of a percent, a measurement error of the current amplification factor .beta. is very large. Also, a parasitic oscillation may occur in the same way in the second prior art dummy bipolar transistor circuit where the collector is connected directly to the power supply pad.
In a third prior art dummy bipolar transistor circuit, operational amplifiers and a current mirror circuit are provided, thereby enabling a measurement of a current amplification factor .beta. without depending on the absolute value of resistors (see JP-A-HEI1-237466).
In the third prior art dummy bipolar transistor circuit, however, the presence of the operational amplifiers and the current mirror circuit reduces the integration. Also, it is difficult to introduce such accurate operational amplifiers into a digital semiconductor integrated circuit. Thus, the third prior art dummy bipolar transistor circuit is difficult in actual practical use.